A device of the above-mentioned type is known from DE-B2-2 357 105. This known device comprises a ring body consisting of a magnetostrictive material and having an annular cavity for the receipt of a measuring winding. The device is based on the characteristic of a magnetostrictive material that it changes its permeability under mechanical load. This results in changes of the magnetic flux through the measuring winding, i.e. to changes of the AC resistance of the measuring winding. The object of the invention according to said publication is to provide a device for magneto-elastic measurement of compressive forces wherein the ring body in case of pressure influence is exclusively or at least to such an extent only compressively loaded that a harmful influence on the accuracy of the measuring result because of occurring tensile stresses in the material is eliminated, so that the measurement becomes as accurate as possible. This object is achieved in that each front surface of the ring body has a ring groove which is aligned with the cavity and having a width essentially equal to the width of the cavity, so that on each front surface there are formed two circular projections of which each lengthens one of the side walls of the ring body and which together form the force-receiving surface. Thus, it is here the question of a special design of the front surfaces of the ring body.
In the known device the material choice is strongly limited because of the fact that the material of the ring body must be magneto-elastic. The measuring winding must be relatively large since it has to magnetize a large "iron" volume with alternating current which must have a relatively low frequency with a view to reduced eddy-current losses. In addition, the device will not be suitable for measuring preloads in critical bolt connections since it is not constructed to stand up to the high stresses occurring in the material in these critical applications.